what is the minimum amount of space you should leave behind the m50 and m40. I am trying to decide between the two and my limited space might restrict me to only one of them.
Both the M40 and M50 only require 2 - 3 inches behind them to operate properly. Depending on your room, having the speakers near the walls and corners may have a tendency to increase certain frequencies at your listening position. With a large room, small speakers, or certain room cancellations this effect can actually be a positive one. If in your room you seem to be getting too much information between 100 and 300 Hz then the best solution is to move the speakers out from the wall a bit, normally around 12 inches is enough to eliminate or at least drastically reduce this room effect.